This disclosure is directed to a device and methods for a temperature controller that is mounted directly to the object whose temperature the device and methods are controlling.
There are numerous applications in which the temperature of an object or a space must be controlled within a close tolerance. Conventionally, control is performed by a controller connected to a temperature measuring device or temperature sensor. A comparison is made generally between a measured or sensed temperature and a preset temperature. The controller then controls power to a heater or cooler to control the temperature of the object or space to within a predetermined limited range from the set temperature.
One conventional method to achieve temperature control from an attached controller involves applying a bimetallic strip at least as the sensor element. FIG. 1 shows a conventional bimetallic strip controller. The bimetallic sensor 1 is mounted on an object 3. A heater or cooler 2 is provided to heat or cool the object 3. A power supply 4 provides power to the heater 2. The bimetallic strip controller 1 includes a bimetallic strip 5, contacts 6 and an adjustment mechanism 7. The bimetallic strip may also be mounted within a space where the temperature is to be controlled.
As the temperature changes, the bimetallic strip 5 will bend or straighten depending on the temperature of the bimetallic strip 5. Bimetallic strip temperature sensors are designed so that the bimetallic strip 5 is bistable, and has two stable states. Depending upon the temperature and the inherent stress caused by that temperature, the bimetallic strip 5 will alternate between the two states. The advantage of the bistablity is that a deadband forms due to hysterisis and the state does not alternate back and forth at a single temperature set point. The state will change from a first state to a second state at a first temperature, but change from the second state back to the first state at a slightly different second temperature. This deadband between the first and second temperatures means that when the object or space is maintained in a temperature range near the set temperature such that the heater or cooler is not being constantly switched on and off.
Assuming that the object 3 must be held above ambient temperature the heater or cooler 2 is a heater. The bimetallic strip 5 is in close contact with the object 3 and has approximately the same temperature as object 3. When the temperature of object 3 is below a first preset temperature that is, a little above the desired temperature, defined by adjustment mechanism 7, the bimetallic strip is stressed by the low temperature into the first stable state, so that contacts 6 are connected. Power flows from the voltage source 4 to the heater 2. As a result, the object 3 warms up along with the bimetallic strip 5. When the temperature reaches the first preset temperature, the bimetallic strip is stressed by the change in temperature and changed to the second stable state in which the contacts 6 are broken. Power is then disconnected from the heater 2 and the object 3 will cool until the temperature two a second preset temperature, a little below the desired temperature. At which point the bimetallic strip will change back to the first stable state in which contacts 6 are connected and the heater 2 will begin to heat object 3 once more. This process continues keeping the object 3 within the deadband temperature range, around the desired set temperature.
The adjustment mechanism 7 allows the adjustment of the set temperature and the deadband. The adjustment mechanisms 7 of these bimetallic strips 5 usually involve setscrews or dials, and due to the nature of the bimetallic strip 5, it is difficult to precisely set the set temperature and the deadband.
Bimetallic strip controllers are convenient because they are small, mountable to any surface, durable and adaptable to numerous temperature ranges. They are also fairly reliable and inexpensive to produce for a specific application. Further, the bimetallic strip controller and, therefore, the entire control system can be mounted on a surface of the object or within the space to be controlled. All of the elements of the bimetallic strip controller can easily be made to withstand extremes of temperature so the set temperature can vary over a wide range. Further, no external connections or control are required.
Another conventional method to achieve temperature control is to use a solid-state controller. FIG. 2 shows conventional solid-state controller approach. Object 3, heater and cooler 2 and voltage source 4 are the same as that shown in FIG. 1. The temperature of object 3 is sensed by a temperature sensor 12. This temperature sensor 12 might be a thermal couple, a dedicated semiconductor temperature sensor, a thermistor, a resistive temperature-sensing device (RTD) or the like. The temperature sensor 12 is connected by wiring 13 to a temperature controller 15. The temperature controller 15 integrates several components. A controller 10 is used to compare the temperature measured by temperature sensor 12 to some reference 14. The reference 14 is used to derive first and second preset temperatures around a set temperature. When the temperature sensed by temperature sensor 12 is below the first preset temperature, the controller 10 controls a power device 11 to allow current to pass from the voltage source 4 to the heater 2. The power device may be a high-powered transistor thyristor, or TRIAC, or some manner of electromechanical relay to control the power. When the controller 10 measures a temperature above first preset temperature, the controller 10 controls power device 11 to switch preventing current flowing to heater 2 so that the object 3 cools. When the controller 10 measures a temperature below the second preset temperature, the controller 10 controls power device 11 to switch allowing current flowing to heater 2 so that the object 3 heats once more. As with the bimetallic strip controller, this process continues keeping the object 3 within a deadband around the set temperature.
The various components in the controller 15 are in general temperature sensitive but their performance is more susceptible to changes in temperature even to the point of destruction. Therefore, in these kinds of control systems only the temperature sensor 12 is generally placed on the object 3 or in the space, to be controlled (particularly where the object or space temperature are to be controlled to extremes of temperature). Other components are placed in controller 15, which is often remotely placed in an environment that is less extreme than that of object 3. This need to place temperature controller 15 in a less harsh environment than that of object 3 presents many issues with additional wiring, additional casing, and a need to find the less harsh environment.
A considerable advantage of bimetallic strip controllers such as that in FIG. 1, is that the components used to make the bimetallic strip controller can stand an extreme range of temperatures. The bimetallic strip controller can simply be mounted on the object, without the additional wiring and casing.
Bimetallic strips are still widely used because of the above issues with remotely placing the solid-state controller 15. Bimetallic strips controllers, however, have problems of their own. If the currents to be controlled are large, and the heater or cooler load is inductive, large sparks are formed as the contacts are made and broken to switch on and off the heater or cooler. This rapidly destroys the contacts. There are many locations where the environment may contain explosive vapors or liquids, and these sparks may ignite fires or explosions. Further, to adjust the set point temperature and the deadband for a bimetallic strip is not trivial and often involves a number of set screws or dials that change the point at which the bimetallic strip will change from the first state to the second state. Adjusting the bimetallic strip device is, also difficult, as there is often not a good correlation between the set point of any individual screw or dial and the set point temperature or deadband. Further, because the accuracy of the set point temperature and the width of the deadband are imprecise, the width of the deadband often cannot be made relatively small. Conversely, even if the width of the deadband could be reduced, the width determines how often the contacts of the controller switch and therefore how fast the contacts wear.
Solid-state base controllers on the other hand can be very accurately controlled and the predetermined range relatively easily adjusted. Further, if a solid-state device controls the power, then there is no disadvantage for example with regard to contact wear, or the generation of sparks, that may cause ignition.
Solid-state controllers have one further advantage, which is that increasingly precise and sophisticated methods for controlling temperature can be implemented. For example, a heater or cooler may be varyingly controlled such that it is effectively maintained in a third state between an off and an on state. A power control device of a solid-state controller may operate in a pulse width modulation technique in which the heater or cooler is rapidly switched on and off, the ratio of on time to off time determining the heating or cooling. Because any heater or cooler can be operated somewhere between fully on and fully off, the solid-state controller can implement techniques such as a proportional integral differential (PID) algorithm to control the temperature. These algorithms lead to more accurate and stable temperature control because they have no deadband. Further, these algorithms stabilize temperature fast when the device is first switched on, and is moving towards a set temperature, or if external conditions around the object or space to be controlled change so that more or less heat is required.